Callas working on aria's ending in opera house (2,5)
I believe the answer is:
la scala
'opera house' is the definition.
(I've seen this before)
'callas working on aria's ending' is the wordplay.
'working' indicates anagramming the letters.
'on' means one lot of letters goes inside another (as in clothing \'on\' a person).
'ending' suggests the final letters.
The final letter of 'aria' is 'a'.
'callas' with letters rearranged gives 'lscala'.
'lscala' going around 'a' is 'LA SCALA'.
'in' is the link.
(Other definitions for la scala that I've seen before include "A Callas may find way to Milan's famous opera house" , "noted venue?" , "Italian opera house" , "Milanese opera house" , "where Toscanini was musical director" .)